[LLVMdev] StructType field names

[Vikram S. Adve]

If you want to give programmers feedback at the source level, CLang is  
almost certainly a better choice.

--Vikram
Associate Professor, Computer Science
University of Illinois at Urbana-Champaign
http://llvm.org/~vadve



On Mar 27, 2009, at 10:25 AM, Anthony Danalis wrote:

> Thanks Luke,
>
> I was afraid that this would be the case.  I can see why this
> information is useless for most people/optimizations.
> However, it is still useful if you are writing an analysis pass that
> is supposed to tell the developer things about her code, and you want
> the output messages to be human readable.
>
> Regarding the "x" or "z" dilemma, I noticed that when you print the
> gep instruction, the generated string says "struct A" (or whatever the
> struct name really is), so all I would need beyond that is a list of
> structs and the names of their fields.  I do understand though that
> this is a very low priority feature for anybody to implement, and I
> since do not want to get involved with the front-end myself I will
> probably write an external perl script that given a struct name and an
> offset it returns a field name.
>
> My question for the list is this:
> Are there any optimizations that could change a struct type?  That is,
> could llvm deduce that the first field of a given struct is never  
> used/
> defined and change the struct type (and all the offsets wherever they
> appear) so that this field does not exist anymore?  In other words, is
> it safe to do an external, trivial mapping from struct name+offset to
> field name, or are there optimizations that would break it?
>
> thanks,
> Anthony
>
> On Mar 26, 2009, at 6:25 PM, Luke Dalessandro wrote:
>
>> Anthony Danalis wrote:
>>> Hello,
>>>
>>> I'm trying to construct a string like "a[1][x].y" in an optimization
>>> pass by digging deeper and deeper into a GetElementPtrInst.  I can
>>> successfully deal with the array/pointer part, but when it comes to
>>> the structure field name "y", I cannot figure out how to get  
>>> anything
>>> but the index into the structure.  Is there a way to do that, or is
>>> this information discarded by llvm?
>>
>> LLVM uses structural equivalence and uniqueing for types, so the
>> information that you are looking for isn't really relevant. For
>> example,
>> if I have the following two types:
>>
>> struct A {
>>   int x, y;
>> };
>>
>> struct B {
>>   int z, w;
>> };
>>
>> They'll both become something like:
>>
>> { int, int }
>>
>> As you noticed, all the gep encodes is the offset into the type, so
>> all
>> you know is that the access is to the first/second field of the
>> type. It
>> wouldn't make sense to have a name for it because you wouldn't know  
>> if
>> the first field was called "x" or "z" anyway. I could imagine some
>> front
>> end work that might embed information that you would need to
>> reconstruct
>> this, but I imagine it would take some doing.
>>
>> Hope this helps,
>> Luke
>>
>>>
>>> thanks,
>>> Anthony
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20090327/e8a5f573/attachment.html>